This invention relates to the field of video projection display and in particular to the measurement of photo sensor signals generated by incident projected illumination.
In a projection video display, geometrical raster distortions result from the physical placement of the cathode ray display tubes. Such raster distortions are exacerbated by the use of cathode ray tubes with curved, concave display surfaces and the inherent magnification in the optical projection path. The projected image is composed of three scanning rasters which are required to be in register one with the other on a viewing screen. The precise overlay of the three projected images requires the adjustment of multiple waveforms to compensate for geometrical distortion and facilitate the superimposition of the three projected images. However, manual alignment of multiple waveforms is labor intensive during manufacturing, and without the use of sophisticated test equipment may preclude setup at a user location. Thus an automated convergence system is disclosed which simplifies manufacturing alignment and facilitates user location adjustment. An automated alignment system may employ raster edge measurement at peripheral display screen locations in order to determine raster size and convergence. Such raster edge measurement may be facilitated by a plurality of photo sensors, which to simplify sensor connection and measurement system interface requirements the sensors are coupled in parallel arrangement. However, a parallel coupling arrangement necessitates a measurement control system capable of identifying individual sensors from the plurality.
A projection television display apparatus comprises a display device with an image formed for projection and a display screen for receiving the projected image. A plurality of photo sensors are located adjacent to edges of the display screen and each generates an output signal responsive to an intensity of illumination by the projected image. A detector with controllable detection threshold is coupled to the photo sensors for generating a signal having a first state when the output signal generated by each of the plurality photo sensors is greater than the detection threshold, and a second state when the output signal generated by each of said plurality photo sensors is less than the detection threshold. A controller is coupled to the display device and to the detector for setting the detection threshold for the output signal generated by each one of the plurality of photo sensors. In response to the signal having the first state, the controller moves the image on the display device to sequentially illuminate another one of the plurality of photo sensors.